feat(sandbox): enforce proxy-aware network routing in sandbox (#11113)

## Summary
- expand proxy env injection to cover common tool env vars
(`HTTP_PROXY`/`HTTPS_PROXY`/`ALL_PROXY`/`NO_PROXY` families +
tool-specific variants)
- harden macOS Seatbelt network policy generation to route through
inferred loopback proxy endpoints and fail closed when proxy env is
malformed
- thread proxy-aware Linux sandbox flags and add minimal bwrap netns
isolation hook for restricted non-proxy runs
- add/refresh tests for proxy env wiring, Seatbelt policy generation,
and Linux sandbox argument wiring

codex-rs/core/tests/suite/seatbelt.rs

@@ -190,6 +190,7 @@ assert os.read(master, 4) == b"ping""#
         &policy,
         sandbox_cwd.as_path(),
         StdioPolicy::RedirectForShellTool,
+        None,
         HashMap::new(),
     )
     .await
@@ -242,6 +243,7 @@ async fn java_home_finds_runtime_under_seatbelt() {
         &policy,
         sandbox_cwd.as_path(),
         StdioPolicy::RedirectForShellTool,
+        None,
         env,
     )
     .await
@@ -298,6 +300,7 @@ async fn touch(path: &Path, policy: &SandboxPolicy) -> bool {
         policy,
         sandbox_cwd.as_path(),
         StdioPolicy::RedirectForShellTool,
+        None,
         HashMap::new(),
     )
     .await